Method for operating a power dissipating unit in a wind turbine

ABSTRACT

A variable speed wind turbine is provided. The wind turbine comprises a generator, a power converter for converting at least a portion of electrical power generated by the generator wherein the power converter comprises a generator side converter, a grid side converter and a DC (direct current) link therebetween, a power dissipating unit operatively coupled to the DC link and a controller. The controller is adapted to determine a DC link voltage error signal, the DC link voltage error signal being the difference between a function of an actual DC link voltage and a function of a predefined reference DC link voltage, determine a DC link error power based on the DC link voltage error signal, determine a feed forward power and generate a duty ratio for operating the power dissipating unit based on the DC link error power and the feed forward power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/976,292,filed Dec. 22, 2010, which claims priority to Danish Patent ApplicationNo. PA 2010 70004 filed on Jan. 4, 2010 and claims the benefit of U.S.Provisional Application No. 61/292,995 filed on Jan. 7, 2010, thecontent of each is incorporated by reference herein in its entirety forall purposes.

FIELD OF THE INVENTION

The present invention relates generally to a wind turbine, and inparticular, to a method for operating a power dissipating unit in a windturbine.

BACKGROUND OF THE INVENTION

A wind turbine is an energy conversion system which converts kineticwind energy into electrical energy for utility power grids.Specifically, wind is applied to wind turbine blades of the wind turbineto rotate a rotor. The mechanical energy of the rotating rotor in turnis converted into electrical energy by an electrical generator. Becausewind speed fluctuates, the force applied to the wind blades and hencethe rotational speed of the rotor can vary. Power grids however requirea constant frequency electrical power to be provided by the windturbine.

One type of wind turbine that provides constant frequency electricalpower is a fixed-speed wind turbine. This type of wind turbine requiresa generator rotor that rotates at a constant speed. A disadvantage ofsuch fixed-speed wind turbine is that it does not harness all of thewind's energy at high speeds and must be disabled at low wind speeds.Another type of wind turbine is a variable speed wind turbine. This typeof wind turbine allows the generator to rotate at variable speeds toaccommodate for fluctuating wind speeds. By varying the rotating speedof the generator rotor, energy conversion can be optimized over abroader range of wind speeds.

A variable speed wind turbine usually includes a power converter havinga generator side converter coupled to a grid side converter via a directcurrent (DC) link. The generator side converter regulates the power ofthe generator. This power passes through the DC-link, and is eventuallyfed to the grid through the grid side converter. The same is true forthe Doubly Fed Induction Generator (DFIG) systems where only a portionof the power from the generator passes through the power converter.

Under normal conditions, the electrical power or energy from thegenerator is supplied to the grid through the power converter. In otherwords, the energy captured from the wind by the wind turbine is passedto the grid. Therefore, it can be said that there is power balanceduring normal conditions. However, when there is a sudden wind gustand/or grid fault, this power balance may be disrupted, resulting inmore power being generated than power being supplied to the grid. Suchpower imbalance might lead to undesired tower oscillations, drive traindamage or turbine tripping.

Specifically, the power output of the generator in response to a suddenwind gust can be approximated as ramp input to the power system in thewind turbine with a steep slope. Such load ramping is one of the mostdifficult load behaviors for a control system in the wind turbine. Awind turbine normally handles wind gust by pitching the blades to reducethe speed of rotor as disclosed, for example, in US 2009/0224542 and EP2107236. However, due to the dynamics of a pitch controller, thepitching of the blade may not be fast enough to respond to the suddenwind gust. Hence this results in the sudden increase in the powergenerated by the generator, leading to the undesired tower oscillations,etc as mentioned above.

When there is a grid fault, for example a low voltage event, there is asudden drop in demand for active power from the grid. Since the pitchingof the blades is not able to respond fast enough to reduce powergeneration, there is an imbalance of power in the wind turbine. U.S.Pat. No. 7,411,309 discloses the use of a crowbar circuit during lowvoltage events at the grid. The crowbar circuit is coupled to the DClink between the generator side converter and the grid side converter.When the DC link voltage exceeds a predetermined value (due to gridfault), the crowbar circuit is activated to drain the excess generatorpower, hence lowering the DC link voltage.

The use of a crowbar circuit or a dump load circuit may provide a goodway of dissipating excess power during a power imbalance event. The dumpload circuit is activated by detecting an abnormal increase in the DClink voltage or a sudden drop in grid voltage. However, it may not bethe most effective method to handle power imbalance events such as windgust, or in extreme conditions when wind gust and grid fault happen atthe same time. Moreover in this method, the resistor bank in the dumpload circuit is excessively stressed.

It is thus an object of the invention to provide an improved way ofmanaging excess power generated in the wind turbine in power imbalanceevent.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a variable speed windturbine is provided. The wind turbine comprises a generator, a powerconverter for converting at least a portion of electrical powergenerated by the generator, a power dissipating unit operatively coupledto a DC (direct current) link of the power converter and a controller.The controller is adapted to determine a DC link voltage error signal,the DC link voltage error signal being the difference between a functionof an actual DC link voltage and a function of a predefined reference DClink voltage, determine a DC link error power based on the DC linkvoltage error signal, determine a feed forward power and generate a dutyratio for operating the power dissipating unit based on the DC linkerror power and the feed forward power.

The power converter includes a generator-side converter for convertingat least a portion of AC power from the generator into DC power, agrid-side converter for converting the DC power into AC power havingfixed frequency and a DC link between the generator-side converter andthe grid-side converter.

The generator is an electromechanical machine capable of convertingmechanical energy into electrical energy. The generator used in the windturbine could be any type of generator including but not limited to, apermanent magnet generator, doubly-fed induction generator and squirrelcage induction generator. The electrical power from the generator has avariable frequency due to the variable rotational speed of the rotor. Aportion or all of the electrical energy or power generated by thegenerator is converted by the power converter into a fixed frequencyelectrical power suitable to be supplied to a power grid or a load.

The load may be a DC or an AC (alternating current) load. For supply ofpower to the grid, the power converter converts the electrical powerwith variable frequency into electrical power having a fixed frequencyrequired by the grid. When supplying power to a load, for example a DCload, the power converter converts the electrical power with variablefrequency into a DC power.

The power dissipating unit is coupled to the DC link of the powerconverter. The power dissipating unit is adapted to dissipate any excesspower generated by the generator which can not be given to the grid. Thepower dissipating unit may be a resistor bank and may also be known as achopper resistor.

The controller is adapted to determine the DC link voltage error signal.The DC link voltage error signal is the difference between a function ofthe actual DC link voltage and a function of the predefined reference DClink voltage. The function of the actual DC link voltage and thefunction of the predefined reference DC link voltage refer to anymathematical expression of the DC link voltage. Examples of the functioninclude:

ƒ(X)=aX+b; where a and b are constants, and

ƒ(X)=X², or any form of a polynomial expression,

where X is the actual DC link voltage. The DC link voltage error signalcan be expressed as ƒ₁(X)−ƒ₂(Y), where Y is the predefined reference DClink voltage. The functions ƒ₁ and ƒ₂ may denote the same or differentfunctions. The DC link error power is derived from the DC link voltageerror signal. Based on the DC link error power and the feed forwardpower, the duty ratio for operating the power dissipating unit isdetermined.

The duty ratio refers to the percentage of time period the powerdissipating unit is activated or turned on in one cycle. The duty ratiohas a value from 0 to 1. When the duty ratio is 0, the power dissipatingunit is turned off completely, and when the duty ratio is 1, the powerdissipating unit is turned on for the whole duty cycle. When the dutyratio is 0.7, the power dissipating unit is turned on for 70% of theduty cycle (it is off for 30% of the remaining duty cycle). Theadvantage of using a duty ratio to control the operation of the powerdissipating unit is that only the amount of excess power in the windturbine is dissipated. An effect of dissipating only the amount ofexcess power using such a duty ratio control according to the embodimentis that maximum power is still supplied to the grid. This is in contrastto the prior art where the power dissipating unit is turned on and offbased only on DC link voltage when the DC link voltage rises above apredetermined level. As the method according to the prior art does notknow how much power to dissipate (as controlled by the duty ratio) andis only concerned with maintaining the DC link voltage within thepredetermined level, it tends to dissipate most of the power in thepower dissipating unit. This results in very low or no power beingsupplied to the grid and other turbine utilities. The dissipation ofmost of the power in the power dissipating unit also stresses theresistor banks in the power dissipating unit.

Additionally, the inclusion of the feed forward power in determining theduty ratio results in a fast response in activating the powerdissipating unit when there is power imbalance in the wind turbine.

According to an embodiment, the power dissipating unit comprises atleast a switch and one resistor. The power dissipating unit is turned onby closing the switch. The switch may be a power semiconductor devicesuch as an Integrated Gate Bipolar Transistor (IGBT) which can be turnedon or off by a suitable voltage through a gate driver. In alternativeembodiments, the power dissipating unit may include at least a switchand at least one of a resistor, an inductor or a capacitor.

According to an embodiment, the DC link voltage error signal is thedifference between the squares of the actual DC link voltage and thepredefined reference DC link voltage. Specifically, the DC link voltageerror signal can be expressed as X²−Y², where X is the actual DC linkvoltage and Y is the predefined reference DC link voltage. As describedearlier, the DC link error signal can be the difference between otherfunctions of the actual DC link voltage and the predefined reference DClink voltage in other embodiments, such as X−Y, (X²+1)−(Y²+1), etc.

According to an embodiment, the controller further comprises a PI(Proportional Integral) controller for determining the DC link errorpower based on the DC link voltage error signal. The advantage of usinga PI controller for determining the DC link error power is itssimplicity of implementation. In other embodiments, a P (Proportional)controller or a PID (Proportional Integral Derivative) controller may beused to determine the DC link error power.

According to an embodiment, the controller is further adapted todetermine the feed forward power based on the difference between thepower supplied to the generator side converter and the power transferredby the grid side converter. Under normal conditions, the power suppliedto the generator side converter from the generator and the powertransferred by the grid side converter is approximately the same.Therefore, the feed forward power is approximately zero, assuming zeropower losses. However when there is wind gust and/or grid faults, thepower supplied to the generator side converter exceeds the powertransferred by the grid side converter. Hence, the feed forward powerbecomes non-zero. This non-zero feed forward power in the event of powerimbalance in the wind turbine leads to faster activation of the powerdissipating unit. It is also possible to determine the feed forwardpower based on other factors such as power captured from the wind in analternative embodiment.

According to an embodiment, the controller is adapted to estimate thepower supplied to the generator side converter based on the differencebetween the power extracted from the wind and power losses in thegenerator and in a drive train of the wind turbine. This has theadvantage that the power supplied to the generator side converter fromthe generator can be obtained well in advance. In other embodiments, thepower supplied to the generator side converter is obtained from thephase voltages and currents at the terminals between the generator andthe generator side converter.

According to an embodiment, the controller is adapted to generate theduty ratio by determining power to be dissipated by the powerdissipating unit, determining a maximum power that can be dissipated bythe power dissipating unit, and determining the ratio of the power to bedissipated and the maximum power, thereby obtaining the duty ratio. Thepower to be dissipated by the power dissipating unit includes the DClink error power and the feed forward power. If the power to bedissipated exceeds the maximum amount of power that can be dissipated bythe power dissipating unit, the duty ratio will be 1. In a furtherembodiment, the power dissipating unit is designed such that the maximumamount of power that can be dissipated by the power dissipating unit isalways larger than the power that needs to be dissipated.

According to an embodiment, the controller is adapted to determine powerextracted from the wind, power supplied by the wind turbine and powerloss in the power dissipating unit, determine the difference between thepower extracted from the wind and the sum of the power supplied by thewind turbine and power loss in the power dissipating unit, and activatethe power dissipating unit when the difference in the power exceeds apredefined power difference threshold.

The power supplied by the wind turbine to the grid may also take intoaccount power losses by various components in the turbine. When thedifference between the power extracted from the wind and the sum of thepower supplied by the wind turbine and power loss in the powerdissipating unit exceeds the predefined power difference threshold, thepower dissipating unit is turned on completely for the full time period.In other words, the duty ratio is set to 1. This has the advantage thatit leads to an even faster response of the power dissipating unit underextreme conditions such as severe wind gust/turbulence and/or extremegrid faults, thus avoiding drive train damage, tower oscillations andturbine tripping. It should be noted that the power loss in the powerdissipating unit is only non-zero when the power dissipating unit hasbeen activated or turned on. In other words, when the power dissipatingunit is turned off, the power loss in the power dissipating unit iszero.

According to a second aspect of the invention, a variable speed windturbine is provided. The wind turbine comprises a generator, a powerconverter for converting at least a portion of electrical powergenerated by the generator wherein the power converter comprises agenerator side converter, a grid side converter and a DC linktherebetween, a power dissipating unit operatively coupled to the DClink and a controller. The controller is adapted to determine powerextracted from the wind, power supplied by the wind turbine and powerloss in the power dissipating unit, determine the difference between thepower extracted from the wind and the sum of the power supplied by thewind turbine and power loss in the power dissipating unit, and activatethe power dissipating unit when the power difference exceeds apredefined power difference threshold.

According to an embodiment, the controller is further adapted to set aduty ratio for operating the power dissipating unit to a non-zero valuewhen the predefined power difference threshold is exceeded, therebyactivating the power dissipating unit.

According to a third aspect of the invention, a method for operating apower dissipating unit in a wind turbine is provided. The wind turbinecomprises a power converter for converting at least a portion ofelectrical power generated by a generator. The power converter comprisesa generator side converter, a grid side converter and a DC linktherebetween. The power dissipating unit is operatively coupled to theDC link. The method comprises obtaining a DC link voltage error signal,the DC link voltage error signal being the difference between a functionof an actual DC link voltage and a function of a predefined reference DClink voltage, determining a DC link error power and a feed forwardpower, the DC link error power is determined based on the DC linkvoltage error signal, and generating a duty ratio for operating thepower dissipating unit based on the DC link error power and the feedforward power.

It should be noted that a person skilled in the art would readilyrecognize that any feature described in combination with the firstaspect of the invention could also be combined with the third aspect ofthe invention, and vice versa.

According to a fourth aspect of the invention, a method for operating apower dissipating unit in a wind turbine is provided. The wind turbinecomprises a power converter for converting at least a portion ofelectrical power generated by a generator. The power converter comprisesa generator side converter, a grid side converter and a DC linktherebetween. The power dissipating unit is operatively coupled to theDC link. The method comprises determining power extracted from the wind,power supplied by the wind turbine and power loss in the powerdissipating unit, determining the difference between the power extractedfrom the wind and the sum of the power supplied by the wind turbine andpower loss in the power dissipating unit, and activating the powerdissipating unit when the difference between the power exceeds apredefined difference threshold.

It should be noted that a person skilled in the art would readilyrecognize that any feature described in combination with the secondaspect of the invention could also be combined with the fourth aspect ofthe invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the detaileddescription when considered in conjunction with the non-limitingexamples and the accompanying drawings.

FIG. 1 shows a general structure of a wind turbine.

FIG. 2 shows an electrical system layout of the wind turbine with achopper or power dissipating unit circuit.

FIG. 3 shows a control algorithm for operating the chopper circuit whenthere is power imbalance in the wind turbine according to an embodiment.

FIG. 4 shows a control algorithm for operating the chopper circuit whenthere is extreme power imbalance in the wind turbine according to anembodiment.

FIG. 5 shows an overall control algorithm for operating the choppercircuit. according to an embodiment.

FIG. 6 shows a flow-chart of a method for operating the chopper circuitin the wind turbine according to an embodiment.

FIG. 7 shows a flow-chart of a method for operating the chopper circuitin the wind turbine when there is extreme power imbalance in the windturbine according to an embodiment.

FIG. 8 shows a flow-chart of a method for operating the chopper circuitin the wind turbine according to a further embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a general setup of a wind turbine 1. The wind turbine 1includes a tower 2 having a number of tower sections, a nacelle 3positioned on top of the tower 2, and a rotor 4 extending from thenacelle 3. The tower 2 is erected on a foundation 7 built in the ground.The rotor 4 is rotatable with respect to the nacelle 3, and includes ahub 5 and one or more blades 6. Wind incident on the blades 6 causes therotor 4 to rotate with respect to the nacelle 3. The mechanical energyfrom the rotation of the rotor 4 is converted into electrical energy bya generator (not shown) in the nacelle 3. The electrical energy issubsequently converted into a fixed frequency electrical power by apower converter to be supplied to a power grid. The wind turbine mayalso form part of a wind farm or a wind power plant comprising aplurality of wind turbines. All the electrical power generated by theindividual wind turbines in the wind farm are consolidated and suppliedto the power grid via a Point of Common Coupling (PCC).

Although the wind turbine 1 shown in FIG. 1 has three blades 6, itshould be noted that a wind turbine may have different number of blades.It is common to find wind turbines having two to four blades. The windturbine 1 shown in FIG. 1 is a Horizontal Axis Wind Turbine (HAWT) asthe rotor 4 rotates about a horizontal axis. It should be noted that therotor 4 may rotate about a vertical axis. Such a wind turbine having itsrotor rotates about the vertical axis is known as a Vertical Axis WindTurbine (VAWT). The embodiments described henceforth are not limited toHAWT having 3 blades. They may be implemented in both HAWT and VAWT, andhaving any number of blades 6 in the rotor 4.

FIG. 2 shows an electrical system of the wind turbine having a powerdissipating unit or a chopper circuit 105 according to an embodiment.The electrical system includes a generator 101, a power converter 102and a main transformer 103. The electrical system is connected to apower grid 107. The power converter 102 includes a generator-sideconverter 110 and a grid-side converter 111 connected via a directcurrent (DC) link 112. The DC link 112 includes a DC link capacitor 113.The chopper circuit 105 is connected to the DC link 112, and includes aswitch SW1 and a resistor 114.

The generator 101 converts mechanical energy or power to electricalenergy or power having AC (alternating current) voltage and current(collectively referred to as “AC signals”), and provides the generatedAC signals to the generator-side converter 110. The AC signals from thegenerator have a variable frequency, due to varying wind. Thegenerator-side converter 110 converts or rectifies the AC signals to aDC (direct current) voltage and a DC current (collectively know as “DCsignals”) which are placed on the DC link 112. The grid-side converter111 converts the DC signals on the DC link 112 into fixed frequency ACsignals for the power grid 107. The power comprising the fixed frequencyAC signals at the output of the grid-side converter 111 is stepped up bythe main transformer 103 into a level suitable to be received andtransmitted by the power grid 107.

The operation of the generator-side converter 110 is controlled by agenerator controller 121, and the operation of the grid-side converter111 is controlled by a grid controller 122. The generator controller 121and the grid controller 122 form part of a converter controller 120. Awind turbine controller 123 provides an overall control of the operationof the wind turbine. For example, the wind turbine controller 123 mayreceive information (e.g. wind speed) from external sensors (e.g.anemometer) and provides control signal to a pitch control (not shown)for pitching the blades in order to obtain a desired rotor speed. Thewind turbine controller 123 may also provide control signals to theconverter controller 120 for controlling the converters 110 and 111.

During normal operation of the wind turbine, the electrical powergenerated by the generator is converted by the power converter 102 intopower having fixed frequency AC signals to be supplied to the power grid107. The switch SW1 is open, and hence no power is dissipated in theresistor 114. In other words, assuming no losses, almost all the powergenerated by the generator is supplied to the power grid 107, and thereis “power balance” in the wind turbine. When there is a voltage dip inthe power grid 107 (low voltage event) resulting in decreased activepower transferred by the grid-side converter to be supplied to the gridand/or when there is a sudden wind gust causing a sudden increase in therotational speed of the blades of the wind turbine (wind gust event),the power generated by the generator exceeds the power supplied to thepower grid 107. In other words, there is “power imbalance” in the windturbine. As mentioned earlier, such power imbalance in the wind turbineleads to undesired effects such as tower oscillations, drive traindamage or turbine tripping.

When there is power imbalance in the wind turbine, any excess power thatis not supplied to the grid 107 is dissipated by the resistor 114 in thechopper circuit 105 by closing the switch SW1. According to anembodiment, the operation (opening and closing) of the switch SW1 iscontrolled so that the resistor 114 in the chopper circuit 105 onlydissipates the excess power in the wind turbine. In other words, thechopper circuit 105 is only activated when there is power imbalance inthe wind turbine, and only for a period just enough to dissipate theexcess power. The control of the operation of the switch SW1, and hencethe operation of the chopper circuit 105, shall be described later withreference to FIG. 3.

It should be noted that the electrical system described with referenceto FIG. 2 is only an example of the electrical configuration of the windturbine and only the main components are shown to illustrate theembodiments. The present invention should not be limited to the exactelectrical system configuration shown in FIG. 2. Other electricalconfigurations are possible. For example, a Doubly Fed InductionGenerator (DFIG) configuration may be used in other embodiments. Also,many components in the electrical system of the wind turbine are notshown in FIG. 2. For example, the electrical system may include filtersbetween the generator 101 and the power converter 102, and between thepower converter 102 and the main transformer 103. Also, there may beswitches arranged at various locations for connecting or disconnectingcertain components of the turbine. The resistor 114 in the choppercircuit 105 may include a single resistor or a bank of resistors.

The electrical system shown in FIG. 2 need not be connected to the powergrid 107. It can be connected to an AC or a DC load. If it is connectedto a DC load, the grid-side converter 111 and the transformer 103 may beomitted, and the DC link 112 can be connected directly to the DC load.Alternatively, a DC-to-DC converter may be arranged between the DC link112 and the DC load to step up or step down the DC voltage at the DClink 112 to a suitable DC voltage for the DC load.

FIG. 3 shows a control algorithm for operating the chopper circuit whenthere is power imbalance in the wind turbine according to an embodiment.The voltage at the DC link 210 is used as one of the factors to decidewhether the chopper circuit should be activated. This is because anyelectrical power from the generator if not transferred to the grid orload leads to an increase in the DC-link voltage. Therefore in thiscontrol algorithm, there is no need to detect any power imbalanceseparately as the chopper circuit is activated automatically in theevent of power imbalance as will be evident from the description below.

In the control algorithm of FIG. 3, a function of the predefinedreference DC link voltage 202 and a function of an actual DC linkvoltage 203 are obtained. As mentioned earlier, the function ƒ(X) of theactual DC link voltage (and also the function of the predefinedreference DC link voltage) may be any mathematical expression of the DClink voltage, such as ƒ(X)=X, ƒ(X)=aX+b, or ƒ(X)=X², or any polynomialrelationship, where a and b are constants and X is the actual DC linkvoltage. In this embodiment, the function of the actual DC link voltageis X². The function of the predefined reference DC link voltage alsouses the same function in this embodiment.

The difference between the squares of the actual DC link voltage 203 andthe predefined reference DC link voltage 202 is obtained as a DC linkerror voltage 205. A PI (Proportional Integral) controller 201 receivesthe DC link error voltage 205 as input and outputs a DC link error power206. According to an embodiment, the control algorithm further includesdetermining a feed forward power. The feed forward power is thedifference between the power supplied to the generator side converter207 and the power transferred by the grid side converter 208.

Under normal conditions when there is power balance in the wind turbine,the feed forward power is approximately zero, assuming no power loss.However when there is wind gust and/or grid fault leading to powerimbalance, the feed forward power becomes non-zero. The addition of thefeed forward power to the DC link error power 206 leads to a fastactivation of the chopper circuit.

The power supplied to the generator side converter 207 can be expressedas:P _(M) =V _(am) I _(am) +V _(bm) I _(bm) +V _(cm) I _(cm)  (1)where P_(M) is the power supplied to the generator side converter fromthe generator, V_(am), V_(bm) and V_(cm) are the phase voltages at thegenerator terminals, and I_(am), I_(bm) and I_(cm) are the currentsthrough the generator terminals. The currents I_(am), I_(bm), I_(cm) canbe measured between the power converter and the generator. The voltagesV_(am), V_(bm), V_(cm) can be measured at the generator terminalsdirectly. If it is not possible to measure the voltages V_(am), V_(bm),V_(cm) at the generator terminals, reference voltages at the converterterminals may be used. Using the reference voltages at the converterterminals and measured currents at the generator terminals, the powerfrom the generator in αβ co-ordinate system can be given as:P _(Mαβ)=1.5(v _(α) i _(α) +v _(β) i _(β))  (2)where P_(Mαβ) is P_(M) in the αβ co-ordinate system, v_(α),v_(β) andi_(α),i_(β) are voltages and currents in the α and β co-ordinates,respectively.

In an alternative embodiment, the power supplied to the generator sideconverter from the generator P_(M) is estimated using the followingexpression:P _(M) =P _(Wind) −P _(L,Drivetrain) −P _(L,Generator)  (3)where P_(wind) is power extracted from the wind, P_(L,Drivetrain) arethe losses in the drive train and P_(L,Generator) are the losses in thegenerator. For a given speed and torque, P_(L,Drivetrain) andP_(L,Generator) can be obtained using a lookup table for a given gearboxand generator. The advantage of estimating P_(M) using equation (3) isthat the P_(M) can be obtained faster as compared to using equation (1).

The power transferred by the grid side converter 208 can be expressedas:P _(G) =V _(ag) I _(ag) +V _(bg) I _(bg) +V _(cg) I _(cg)  (4)where P_(G) is the power transferred by grid side converter, V_(ag),V_(bg) and V_(cg) are the voltages at the converter terminals, andI_(ag), I_(bg) and I_(cg) are the currents through the converterterminals. If the voltages at the converter terminals V_(ag), V_(bg),V_(cg) cannot be obtained, for example due to converter switching,reference voltages for the converter may be used instead. Using thereference voltages for the converter and measured currents at theconverter terminals, the power transferred by the grid side converter inαβ co-ordinate system can be given as:P _(Gαβ)=1.5(v _(α) i _(α) +v _(β) i _(β))  (6)where P_(Gαβ) is P_(G) in the αβ co-ordinate system, v_(α),v_(β) andi_(α),i_(β) are voltages and currents in the α and β co-ordinates,respectively.

The total power P_(total) to be dissipated 209 is the sum of the DC linkerror power 206 and the feed forward power (P_(M)−P_(G)). The maximumpower that can be dissipated by the chopper circuit can be determined asfollows:

$\begin{matrix}{P_{\max} = \frac{V_{dc}^{2}}{R_{chopper}}} & (7)\end{matrix}$where P_(max) is the maximum power that can be dissipated by theresistor or resistor bank in the chopper circuit, V_(dc) is the actualDC link voltage, and R_(chopper) is the resistance of the resistor inthe chopper circuit. The resistance value R_(chopper) of the resistor isnormally selected such that the P_(max) is larger than an anticipatedmaximum power that may need to be dissipated in a wind gust and/or gridfault event. In an embodiment, the value of R_(chopper) is chosen suchthat P_(max) is about 10-20% larger than the nominal power rating of theturbine.

The duty ratio for operating the chopper circuit is determined as theratio between the total power to be dissipated P_(total) and the maximumpower P_(max), that is:

$\begin{matrix}{{DR}_{1} = \frac{P_{total}}{P_{\max}}} & (8)\end{matrix}$where DR₁ is the duty ratio. Since the total power P_(total) is alwaysless than the maximum power P_(max), the duty ratio has a value from 0to 1.

Under normal conditions when there is no power imbalance in the windturbine, the voltage at the DC link is regulated by a DC link controllerto a preset DC link voltage. The preset DC link voltage is the voltagelevel which is maintained at the DC link under normal conditions. Thereference DC link voltage V_(dc) _(—) _(ref) is predefined or set to avalue which is higher than this preset DC link voltage. Therefore undernormal conditions, the DC link error power 206 is negative as the DClink voltage (which is regulated to the preset DC link voltage) has avalue lower than the reference DC link voltage. The feed forward power(P_(M)−P_(G)) will be approximately zero, and hence P_(total), isnegative. Accordingly, the duty ratio is zero. The switch SW1 is notturned on, and chopper circuit is not activated.

When there is power imbalance, both the DC link error power 206 and feedforward power (P_(M)−P_(G)) become non-zero. This results in the totalpower to become non-zero. Therefore, the duty ratio will now have anon-zero value from 0 to 1. When the duty ratio has a value of 0.5, thechopper circuit is only activated or turned on for 50% of the time inone duty cycle. Similarly when the duty ratio has a value of 0.3, thechopper circuit is only activated for 30% of the time in one duty cycle.

Accordingly, the chopper circuit is not activated all the time whenthere is power imbalance to dissipate power, but only for an appropriateperiod of time depending on the extent of the power imbalance in thewind turbine as controlled by the duty ratio. Therefore, the efficiencyand effectiveness of the chopper circuit is ensured as only power thatis not supplied to the grid is dissipated. The use of feed forward poweralso ensures fast activation of the chopper circuit in the event ofpower imbalance in the wind turbine. Thus oscillation of the windturbine tower due to sudden wind gust and/or grid fault can be avoidedas the chopper circuit can now be activated quickly.

The control algorithm of FIG. 3 has been described with reference to thefull scale converter based turbine shown in FIG. 2. It should be notedthat the control algorithm described with reference to FIG. 3 is alsoapplicable in a DFIG system. In the full scale converter based turbineshown in FIG. 2, the power transferred by the grid side converter 111 isapproximately the same as the power supplied to the grid 107 if anypower losses between the output of the grid side converter 111 and thegrid 107 is assumed to be negligible. Similarly, the power supplied tothe generator side converter 110 is approximately the same as the powergenerated from the generator 101, assuming negligible power lossesbetween the output of the generator 101 and the generator side converter110.

In a DFIG system, the power supplied to the grid is the sum of the powertransferred by the grid side converter 111 and the power transferredthrough the stator windings. The power generated from the generator 101is the sum of the power supplied to the generator side converter 110 andthe power transferred through the stator windings.

FIG. 4 shows a control algorithm for operating the chopper circuit whenthere is extreme and sudden power imbalance in the wind turbineaccording to an embodiment. The power extracted from the wind P_(wind),the power supplied to the grid or load P_(Grid) and the power loss inthe chopper circuit P_(L,chopper) are obtained. The power differenceP_(diff) between the power extracted from the wind P_(wind), and thepower supplied to the grid P_(Grid) and the power loss in choppercircuit P_(L,chopper) is determined. Specifically, the power differenceis determined using the following expression:P _(diff) =P _(wind) −P _(Grid) −P _(L,chopper)  (9)

The power difference P_(diff) is compared to a predefined powerdifference threshold P_(threshold). If the power difference P_(diff)exceeds the predefined difference threshold P_(threshold), the choppercircuit is turned on, i.e. the duty ratio DR₂ for operating the choppercircuit is set to 1. Otherwise, DR₂ is set to 0. The thresholdP_(threshold) is set to a value such that it is only exceeded when thedifference P_(diff) is large, for example during extreme wind gustand/or extreme fault conditions. P_(threshold) can be stored in a lookuptable for various wind gust and/or extreme fault conditions.

The power from the wind can be determined using the followingexpression:

$P_{wind} = {\frac{1}{2}\rho\;{AV}_{wind}^{3}{C_{p}\left( {\theta,\lambda} \right)}}$where ρ is the air density, A is the rotor area, V_(wind) is the windspeed, C_(p) is rotor power coefficient, θ is the pitch angle and λ isthe tip speed ratio. Assuming constant rotor area A and air density ρ,the power from the wind P_(wind) is proportional to V_(wind) ³C_(p)(θ,λ). The direct use of wind velocity V_(wind) provides a very fast methodof determining whether there is a wind gust event.

As mentioned earlier, in the full scale converter based wind turbinesystem, the power supplied to the grid P_(Grid) is approximately thesame as the power transferred by the grid side converter 111. Therefore,the power supplied to the grid can be determined using equation (4) asdiscussed above. In the DFIG system, the power supplied to the gridP_(Grid) is the sum of the power transferred by the grid side converter111 and the power transferred through the stator windings as the statoris directly coupled to the grid.

The power loss in the chopper circuit P_(L,chopper) is:P _(L,chopper) =V _(dc) ² /R _(chopper) ×DR  (11)where V_(dc) is the DC link voltage and DR is the duty ratio of thechopper. It should be noted that DR may be the same as the duty ratioDR₂ in the embodiment where only the control algorithm in FIG. 4 is usedor is obtained by taking a maximum (MAX) of DR₁ and DR₂ in an embodimentwhere both the control algorithms in FIG. 3 and FIG. 4 are used (seeFIG. 5). When the chopper circuit is not activated, the power loss inthe chopper circuit P_(L,chopper) is zero since the DR is zero.

When there is no wind gust or grid fault, the power supplied to the gridP_(Grid) is approximately the same as the power extracted from the windP_(wind), assuming negligible losses in the drive train. The power lossin the chopper circuit P_(L,chopper) is zero if the chopper circuit isnot activated. At any given time, it can be assumed that the sum of thepower supplied to the grid P_(Grid) and the power loss in the choppercircuit P_(L,chopper) is the total power consumption in the wind turbine(assuming no other losses in the wind turbine drive train). When thereis extreme wind gust and/or grid fault, the power difference P_(diff)between the P_(wind) and the total power consumed P_(Grid) andP_(L,chopper) can become significantly large. This may lead to overspeeding of the generator, tower vibration and/or turbine tripping. Thepower difference P_(diff) is compared to the difference thresholdP_(threshold). This difference threshold P_(threshold) can be tabulatedin a lookup table, and is the limit at which problems such as towervibration and turbine tripping starts to occur. When the differencethreshold P_(diff) exceeds the difference threshold P_(threshold), thechopper circuit is activated to reduce the power difference P_(diff).

Accordingly, the use of the control algorithm in FIG. 4 to control theoperation of the chopper circuit provides a fast and effective way ofactivating the chopper circuit in the event of extreme and sudden windgust and/or grid fault conditions.

In an embodiment, the control algorithm in FIG. 4 is used in conjunctionwith the control algorithm shown in FIG. 3 for operating the choppercircuit in event of power imbalance in the wind turbine. When there iswind gust and/or grid fault, the operation of the chopper circuit iscontrolled by the duty ratio obtained using the control algorithm ofFIG. 3. Under extreme and sudden wind gust or fault conditions, thecontrol algorithm of FIG. 4 is used to activate the chopper circuit.Such arrangement where both the control algorithms are used is shown inFIG. 5.

In FIG. 5, the control algorithm described with reference to FIG. 3 isrepresented as block 300 and the control algorithm described withreference to FIG. 4 is represented as block 301. The outputs of bothblock 300 and block 301 are provided as inputs to a MAX function block302. The output of the MAX function block 302 is provided as the controlsignal for controlling the operation of the chopper circuit.Specifically, the output 303 of the MAX function block 302 is the dutyratio from the control algorithm of FIG. 3 when there is power imbalancein the wind turbine. Under extreme power imbalance, the duty ratio atthe output 303 of the MAX function block 302 gives a value of 1 due tothe output of block 301 being 1. In other words, as long as one of thecontrol algorithms gives a non-zero duty ratio, the chopper circuit isactivated.

It should be noted that the configuration in FIG. 5 is merely anillustrative example on how the control algorithms shown in FIG. 3 andFIG. 4 can be used in conjunction with each other. Other types ofconfigurations, for example taking an OR of the outputs of blocks 300and 301, are possible in other embodiments. The control algorithmsdescribed above with reference to FIG. 3 and FIG. 4 may be implementedin the converter controller 120 and/or the wind turbine controller 123of FIG. 2. It is also possible to implement the control algorithms usingan independent and/or separate controller (not shown in FIG. 2). Itshould also be noted that it is possible to use only one of the controlalgorithms described with reference to FIG. 3 or FIG. 4 to control thechopper circuit in other embodiments.

FIG. 6 shows a flow-chart of a method for operating the powerdissipating unit in the wind turbine according to an embodiment. Step400 includes obtaining a DC link voltage error signal. The DC linkvoltage error signal is the difference between a function of the actualDC link voltage and a function of the predefined reference DC linkvoltage. As mentioned earlier, the function of the actual and predefinedreference DC link voltage may include any mathematical expressionrelating to the DC link voltage. In an embodiment, the function is thesquares of the actual DC link voltage and the predefined reference DClink voltage.

Step 410 includes determining the DC link error power and the feedforward power. The DC link error power is determined based on the DClink voltage error signal. As mentioned earlier, the DC link error powermay be determined using the PI controller with the DC link voltage errorsignal as an input. In an embodiment, the feed forward power includesthe difference between the power supplied to the generator sideconverter and the power transferred by the grid side converter. Step 420includes generating the duty ratio for operating the power dissipatingunit. In an embodiment, the duty ratio is used to operate the powerdissipating unit. The chopper circuit as described with reference toFIG. 2 earlier is an example of the power dissipating unit. The dutyratio is generated based on the DC link error power and the feed forwardpower. In an embodiment, the duty ratio is the ratio between the powerto be dissipated by the power dissipating unit and the maximum power thepower dissipating unit can dissipate. The power to be dissipated is thesum of the DC link error power and the feed forward power in anembodiment. Steps 400 to 420 are then repeated, so that the duty ratiois constantly being updated.

FIG. 7 shows a flow-chart of a method for operating the chopper circuitin the wind turbine when there is extreme power imbalance in the windturbine according to an embodiment. Step 500 includes determining thepower extracted from the wind, the power supplied by the wind turbineand the power loss in the power dissipating unit. The power dissipatingunit may be a chopper circuit in an embodiment. As described earlier,the power from the wind can be determined using equation (10) in anembodiment. The power loss in the power dissipating unit may bedetermined using equation (11). The power loss in the power dissipatingunit is zero if the chopper circuit is not activated.

Step 510 includes determining whether the power difference between thepower extracted from the wind, and the sum of the power supplied to thegrid and the power loss from chopper circuit exceeds the powerdifference threshold. If the power difference exceeds the powerdifference threshold, the duty ratio is set to 1 at step 520. Otherwise,the duty ratio is set to 0 (also at step 520). Steps 500 to 520 are thenrepeated, so that the duty ratio is constantly being updated.

FIG. 8 shows a flow-chart of a method for operating the powerdissipating unit in the wind turbine according to a further embodiment.In this embodiment, the methods as defined in steps 400 to 420 and steps500 to 520 are concurrently used for controlling the operation of thepower dissipating unit. Steps 400 to 420 have already been describedwith reference to FIG. 6 and steps 500 to 520 have already beendescribed with reference to FIG. 7. Steps 400 to 420 and steps 500 to520 are repeated to constantly update the duty ratio.

Step 540 includes determining a maximum of the duty ratios from step 420and step 520, and activating the power dissipating unit in step 550based on the maximum of the two duty ratios. It should be noted that thepower dissipating unit is only activated when the maximum of the dutyratios has a non-zero value.

It should be emphasized that the embodiments described above arepossible examples of implementations which are merely set forth for aclear understanding of the principles of the invention. The personskilled in the art may make many variations and modifications to theembodiment(s) described above, said variations and modifications areintended to be included herein within the scope of the following claims.

1. A variable speed wind turbine comprising: a generator; a powerconverter for converting at least a portion of electrical powergenerated by the generator, the power converter comprising a generatorside converter, a grid side converter and a DC (direct current) linktherebetween; a power dissipating unit operatively coupled to the DClink; and a controller, wherein the controller is adapted to: determinepower extracted from the wind, power supplied by the wind turbine andpower loss in the power dissipating unit; determine the differencebetween the power extracted from the wind and the sum of the powersupplied by the wind turbine and the power loss in the power dissipatingunit; and activate the power dissipating unit when the differencebetween the power exceeds a predefined power difference threshold. 2.The variable speed wind turbine according to claim 1, wherein thecontroller is further adapted to set a duty ratio for operating thepower dissipating unit to a non-zero value when the predefined powerdifference threshold is exceeded, thereby activating the powerdissipating unit.
 3. The variable speed wind turbine according to claim1, wherein the controller is adapted to determine the power extractedfrom the wind according to the following expression:$P_{wind} = {\frac{1}{2}\rho\;{AV}_{wind}^{3}{C_{p}\left( {\theta,\lambda} \right)}}$where ρ is air density, A is rotor area of the generator, V_(wind) isspeed of the wind, C_(p) is a rotor power coefficient, θ is pitch angleand λ is tip speed ratio of the generator.
 4. The variable speed windturbine according to claim 1, wherein the controller is adapted todetermine the power supplied by the wind turbine according to thefollowing expression:P _(G) =V _(ag) I _(ag) +V _(bg) I _(bg) +V _(cg) I _(cg) where P_(G) ispower transferred by the grid side converter, V_(ag), V_(bg) and V_(cg)are voltages at terminals of the grid side converter, and I_(ag), I_(bg)and I_(cg) are currents through terminals of the grid side converter. 5.The variable speed wind turbine according to claim 1, wherein thecontroller is adapted to determine the power loss in the powerdissipating unit according to the following expression:P _(L,chopper) =V _(dc) ² /R _(chopper) ×DR where V_(dc) is DC linkvoltage, R_(chopper) is resistance of a resistor in the powerdissipating unit, and DR is a duty ratio of the power dissipating unit.6. The variable speed wind turbine according to claim 1, wherein thepower dissipating unit comprises at least a switch and one resistor. 7.A method for operating a power dissipating unit in a wind turbine, thewind turbine comprises a power converter for converting at least aportion of electrical power generated by a generator wherein the powerconverter comprises a generator side converter, a grid side converterand a DC (direct current) link therebetween, the power dissipating unitis operatively coupled to the DC link, the method comprising:determining power extracted from the wind, power supplied by the windturbine and power loss in the power dissipating unit; determining thedifference between the power extracted from the wind and the sum of thepower supplied by the wind turbine and the power loss in the powerdissipating unit; and activating the power dissipating unit when thedifference between the power exceeds a predefined power differencethreshold.
 8. The method according to claim 7, further comprisingsetting a duty ratio for operating the power dissipating unit to anon-zero value when the predefined power difference threshold isexceeded, thereby activating the power dissipating unit.